Microelectromechanical Systems (MEMS) are among the most significant technological advances of the last two decades. Bulk micromachining and surface micromachining have become primary processing techniques for fabrication of MEMS devices derived from Si and Si-based materials. However, there is a major drawback associated with the bulk and surface micromachining: low geometric aspect ratio, which severely limits their further applications. To meet the critical need for high-aspect-ratio microdevices, the LIGA (LIthographie-Galvanoformung-Abformung) technique, based on deep X-ray lithography and electroplating, has been developed and studied. The LIGA process is capable of producing microdevices a few centimeters high with an aspect ratio of more than 100, which is particularly advantageous for microdevices such as micromotors and micropumps since the high aspect ratios allow the generation of high torque. In addition, the LIGA process also allows one to use conventional materials such as metals for the MEMS architectures.
It is increasingly important to fabricate LIGA microdevices capable of operating in harsh environments, such as high contact stresses (microgear sets), high temperatures (microcombustion chambers), and corrosive environments (microheat exchangers or microcatalytic converters). Nonetheless, most LIGA microdevices are manufactured out of Ni, Cu, Fe or their alloys, whose performance would be significantly degraded in these demanding situations, even to the extent of total failure. Recently, surface treatment by coatings has been recognized as an effective method to alleviate this serious problem and substantially prolong the lifetime of LIGA microdevices. The characteristics of an ideal protective coating for LIGA microdevices are: low wear rate, low coefficient of friction, low thermal conductivity, strong adhesion, chemical inertness, and high temperature stability. Protective coatings with the potential to form a lubricating film on their surfaces (self-lubricating) are particularly desirable for reducing friction and wear in LIGA microdevices.
Diamond-like carbon (DLC) films have been explored for this use, however, a high compressive residual stress of several GPa usually develops in DLC films, causing delamination of films with thickness of greater than 100 nm. Moreover, DLC films are thermally unstable at temperatures above 723 K, which render them unsuitable for high temperature applications. Low-surface-energy, hydrophobic polymeric coatings are promising for minimizing stiction and friction, but they do not improve the wear resistance of LIGA microdevices because of their comparatively low hardness, and they fail at temperatures only moderately higher than ambient.